Devices for dispensing volatile liquids as vapours are well known in the art and conventionally comprise a reservoir housing for containing the volatile liquid and an element for releasing volatile liquid vapours into the atmosphere.
All such prior art devices fall into one or another of three basic groups:
(1) Devices wherein the reservoir housing comprises the element for releasing the volatile liquid vapours to the surrounding atmosphere, PA1 (2) devices wherein the reservoir housing is relatively rigid and the element for releasing volatile liquid vapours is displaceable relative to said reservoir housing, and PA1 (3) devices wherein the reservoir housing is relatively rigid and the element for releasing volatile liquid vapours is fixed relative to said reservoir housing.
The present invention is concerned only with group (3) devices and all groups (1) and (2) dispensing devices are disclaimed herefrom.
Devices falling into group (3) are marketed in many different forms and conventionally such devices comprise a substantially rigid reservoir housing, for containing the volatile liquid, closed at one end by a vapour-permeable element. Such a device is, hereinafter, referred to as "a device of the type defined".
Devices of the type defined are marketed in many different shapes and sizes and can be used in different ways.
The U.S. Pat. No. 4,753,389 discloses a vaporizing device of the type defined (see FIG. 5) in which the reservoir housing comprises a tubular element, closed at one end by a rigid wall and at its other end by a vapour-permeable element. The reservoir is charged with a volatile liquid and, in use, the device is located with the vapour-permeable element uppermost, whereby a plenum chamber or space is formed between the volatile liquid and the vapour-permeable element. The volatile liquid evaporates into said plenum and is released through the vapour-permeable element to the surrounding atmosphere.
The European Patent Publication No. 0218892 discloses a vaporizing device of the type defined comprising a reservoir housing in the form of a tubular element, closed at one end by a rigid wall and closed at its other end by a vapour-permeable, liquid-impermeable element. The reservoir is charged with a volume of the volatile liquid to be dispensed and, in use, the device is positioned with the said element lowermost, whereupon the volatile liquid wets the inner surface of the vapour-permeable, liquid-impermeable element and evaporates through the said element to escape from the external surface of said element to the surrounding atmosphere.
A third dispensing arrangement of the type defined is disclosed in British Patent Publication No. 2194889B and wherein a housing, in the form of a tubular element, is closed at one end by a rigid wall and at the other end by a vapour-permeable, liquid-impermeable element.
In this third mode of use (see FIG. 1 of the publication) the housing rests on its cylindrical wall so that the said element is substantially vertical. A head space above the free surface of the volatile liquid in the housing is charged with evaporated volatile liquid and the vapours escape through the said element above the free surface of the volatile liquid, whilst liquid entering the element below the free surface of the liquid is vaporized and passes through the said element below the free surface of the liquid.
It should be noted that in all three of the above recited examples, and in most commercially available devices of the type defined, the cross-sectional area of the reservoir is substantially constant at right angles to the plane of the vapour-permeable element and this cross-sectional area is substantially equal to the cross-sectional area of the vapour-permeable element exposed to the volatile material, be this in liquid or vapour form.
As each volatile liquid capable of being dispensed as a vapour will have a rate of vaporization, and a desired rate of release of vapours, individual to that liquid the only variables available in the prior art devices for varying the rate of vaporization and the rate of vapour release is to vary the size of the reservoir housing, which leads to a wide variety of differently sized housings, and/or varying the formulation of the volatile liquid to be contained in the reservoir housing, and which leads to larger volumes of liquid to be contained in the reservoir housing and often the introduction of unnecessary and undesirable impurities in the contained volatile liquids.
A further problem with many prior art devices of the type defined, and in particular the form of device disclosed in the U.S. Pat. No. 4,753,389 and the British Patent Publication No. 2194889B, arises in that many of the volatile liquids dispensable as vapours comprise complex blends of volatile ingredients. With a gas volume above the free surface of the liquid the more volatile of said ingredients can vaporize into the said gas volume more readily than the less volatile materials and can pass through the porous element above so that the composition of the vapours can vary throughout the life of the device.
The present invention seeks to provide a device of the type defined and for which the rate of vaporization of the volatile liquid can be adjusted without varying the dimensions of the reservoir housing.